Philosophical Studies

by G. E. Moore

He proposes, first of all, by way of answer to it, a rule, which may, I think, be expressed as follows: No man, he says, can ever know any matter of fact, which he has not himself observed, unless he can know that it is connected by "the relation of cause and effect," with some fact which he has observed. And no man can ever know that any two facts are connected by this relation, except by the help of his own past experience. In other words, if I am to know any fact, A, which I have not myself observed, my past experience must give me some foundation for the belief that A is causally connected with some fact, B, which I have observed. And the only kind of past experience which can give me any foundation for such a belief is, Hume seems to say, as follows: I must, he says, have found facts like A "constantly conjoined" in the past with facts like B. This is what he says; but we must not, I think, press his words too strictly. I may, for instance, know that A is probably a fact, even where the conjunction of facts like it with facts like B has not been quite constant. Or instead of observing facts like A conjoined with facts like B, I may have observed a whole series of conjunctions—for instance, between A and C, C and D, D and E, and E and B; and such a series, however long, will do quite as well to establish a causal connection between A and B, as if I had directly observed conjunctions between A and B themselves. Such modifications as this, Hume would, I think, certainly allow. But, allowing for them, his principle is, I think, quite clear. I can, he holds, never know any fact whatever, which I have not myself observed, unless I have observed similar facts in the past and have observed that they were "conjoined" (directly or indirectly) with facts similar to some fact which I do now observe or remember. In this sense, he holds, all our knowledge of facts, beyond the reach of our own observation, is founded on experience.

This is Hume's primary principle. But what consequences does he think will follow from it, as to the kind of facts, beyond our own observation, which we can know? We may, I think, distinguish three entirely different views as to its consequences, which he suggests in different parts of his work.

In the first place, where he is specially engaged in explaining this primary principle, he certainly seems to suppose that all propositions of the kind, which we assume most universally in everyday life, may be founded on experience in the sense required. He supposes that we have this foundation in experience for such beliefs as that "a stone will fall, or fire burn"; that Julius Caesar was murdered; that the sun will rise to-morrow; that all men are mortal He speaks as if experience did not merely render such beliefs probable, but actually proved them to be true. The "arguments from experience" in their favour are, he says, such as "leave no room for doubt or opposition." The only kinds of belief, which he definitely mentions as not founded on experience, are "popular superstitions" on the one hand, and certain religious and philosophical beliefs, on the other. He seems to suppose that a few (a very few) religious beliefs may, perhaps, be founded on experience. But as regards most of the specific doctrines of Christianity, for example, he seems to be clear that they are not so founded. The belief in miracles is not founded on experience; nor is the philosophical belief that every event is caused by the direct volition of the Deity. In short, it would seem, that in this doctrine that our knowledge of unobserved facts is confined to such as are "founded on experience," he means to draw the line very much where it is drawn by the familiar doctrine which is called "Agnosticism." We can know such facts as are asserted in books on "history, geography or astronomy," or on "politics, physics and chemistry," because such assertions may be "founded on experience"; but we cannot know the greater part of the facts asserted in books "of divinity or school metaphysics," because such assertions have no foundation in experience.

This, I think, was clearly one of Hume's views. He meant to fix the limits of our knowledge at a point which would exclude most religious propositions and a great many philosophical ones, as incapable of being known; but which would include all the other kinds of propositions, which are most universally accepted by common-sense, as capable of being known. And he thought that, so far as matters of fact beyond the reach of our personal observation are concerned, this point coincided with that at which the possibility of "foundation on experience" ceases.

But, if we turn to another part of his work, we find a very different view suggested. In a quite distinct section of both his books, he investigates the beliefs which we entertain concerning the existence of "external objects." And he distinguishes two different kinds of belief which may be held on this subject. "Almost all mankind, and philosophers themselves, for the greatest part of their lives," believe, he says, that "the very things they feel and see" are external objects, in the sense that they continue to exist, even when we cease to feel or see them. Philosophers, on the other hand, have been led to reject this opinion and to suppose (when they reflect) that what we actually perceive by the senses never exists except when we perceive it, but that there are other external objects, which do exist independently of us, and which cause us to perceive what we do perceive. Hume investigates both of these opinions, at great length in the Treatise, and much more briefly in the Enquiry, and comes to the conclusion, in both books, that neither of them can be "founded on experience," in the sense he has defined. As regards the first of them, the vulgar opinion, he does seem to admit in the Treatise that it is, in a sense, founded on experience; but not, he insists, in the sense defined. And he seems also to think that, apart from this fact, there are conclusive reasons for holding that the opinion cannot be true. And as regards the philosophical opinion, he says that any belief in external objects, which we never perceive but which cause our perceptions, cannot possibly be founded on experience, for the simple reason that if it were, we should need to have directly observed some of these objects and their "conjunction" with what we do perceive, which ex hypothesi, we cannot have done, since we never do directly observe any external object.

Hume, therefore, concludes, in this part of his work, that we cannot know of the existence of any "external object" whatever. And though in all that he says upon this subject, he is plainly thinking only of material objects, the principles by which he tries to prove that we cannot know these must, I think, prove equally well that we cannot know any "external object" whatever—not even the existence of any other human mind. His argument is: We cannot directly observe any object whatever, except such as exist only when we observe them; we cannot, therefore, observe any "constant conjunctions" except between objects of this kind: and hence we can have no foundation in experience for any proposition which asserts the existence of any other kind of object, and cannot, therefore, know any such proposition to be true. And this argument must plainly apply to all the feelings, thoughts and perceptions of other men just as much as to material objects. I can never know that any perception of mine, or anything which I do observe, must have been caused by any other man, because I can never directly observe a "constant conjunction" between any other man's thoughts or feelings or intentions and anything which I directly observe: I cannot, therefore, know that any other man ever had any thoughts or feelings—or, in short, that any man beside myself ever existed. The view, therefore, which Hume suggests in this part of his work, flatly contradicts the view which he at first seemed to hold. He now says we cannot know that a stone will fall, that fire will burn, or the sun will rise to-morrow. All that I can possibly know, according to his present principles, is that I shall see a stone fall, shall feel the fire burn, shall see the sun rise to-morrow. I cannot even know that any other men will see these things; for I cannot know that any other men exist. For the same reason, I cannot know that Julius Caesar was murdered, or that all men are mortal. For these are propositions asserting "external" facts—facts which don't exist only at the moment when I observe them; and, according to his present doctrine, I cannot possibly know any such proposition to be true. No man, in short, can know any proposition about "matters of fact" to be true, except such as merely assert something about his own states of mind, past, present or future—about these or about what he himself has directly observed, is observing, or will observe.

Here, therefore, we have a very different view suggested, as to the limits of human knowledge. And even this is not all. There is yet a third view, inconsistent with both of these, which Hume suggests in some parts of his work.

So far as we have yet seen, he has not in any way contradicted his original supposition that we can know some matters of fact, which we have never ourselves observed. In the second theory, which I have just stated, he does not call in question the view that I can know all such matters of fact as I know to be causally connected with facts which I have observed, nor the view that I can know some facts to be thus causally connected. All that he has done is to question whether I can know any external fact to be causally connected with anything which I observe; he would still allow that I may be able to know that future states of my own, or past states, which I have forgotten, are causally connected with those which I now observe or remember; and that I may know therefore, in some cases, what I shall experience in the future, or have experienced in the past but have now forgotten. But in some parts of his work he does seem to question whether any man can know even as much as this: he seems to question whether we can ever know any fact whatever to be causally connected with any other fact. For, after laying it down, as we saw above, that we cannot know any fact, A, to be causally connected with another, B, unless we have experienced in the past a constant conjunction between facts like A and facts like B, he goes on to ask what foundation we have for the conclusion that A and B are causally connected, even when we have in the past experienced a constant conjunction between them. He points out that from the fact that A has been constantly conjoined with B in the past, it does not follow that it ever will be so again. It does not follow, therefore, that the two really are causally connected in the sense that, when the one occurs, the other always will occur also. And he concludes, for this and other reasons, that no argument can assure us that, because they have been constantly conjoined in the past, therefore they really are causally connected. What, then, he asks, is the foundation for such an inference? Custom, he concludes, is the only foundation. It is nothing but custom which induces us to believe that, because two facts have been constantly conjoined on many occasions, therefore they will be so on all occasions. We have, therefore, no better foundation than custom for any conclusion whatever as to facts which we have not observed. And can we be said really to know any fact, for which we have no better foundation than this? Hume himself, it must be observed, never says that we can't. But he has been constantly interpreted as if the conclusion that we can't really know any one fact to be causally connected with any other, did follow from this doctrine of his. And there is, I think, certainly much excuse for this interpretation in the tone in which he speaks. He does seem to suggest that a belief which is merely founded on custom, can scarcely be one which we know to be true. And, indeed, he owns himself that, when he considers that this is our only foundation for any such belief, he is sometimes tempted to doubt whether we do know any fact whatever, except those which we directly observe. He does, therefore, at least suggest the view that every man's knowledge is entirely confined to those facts, which he is directly observing at the moment, or which he has observed in the past, and now remembers.